Capacitor choices in power-related applications

While capacitor versions have not changed much, there are a number of new power applications, where they are now used, such as alternative energy, electric vehicles (EVs), energy storage and more. This article will compare the various technologies and consider some usage examples.

Have Aluminum electrolytics had their day?

Cost is a major driver in many modern systems and Aluminum electrolytics continue to be lower cost than equivalent film capacitors, making them popular in bulk energy storage applications. The notion that they have lower life and reliability can be dispelled by derating them appropriately for the intended application. Also, they remain, in general, able to offer better energy storage per unit volume than film types – with the possible exception of specialist types such as segmented high-crystalline metallized polypropylene. Aluminum electrolytics perform better at the elevated temperatures found in many applications, especially with regard to parameters such as the ripple current rating.

Where is Film better?

Film can deliver far lower ESR ratings, dramatically improving ripple current performance and can offer better surge ratings. Due to their construction, they also offer better reliability as they can ‘self-heal’ after stress, depending on the amount and how often the stress is repeated. In EVs there is no ‘hold-up’ requirement so film capacitors are ideal here and, as battery stacks increase and bus voltages rise, the voltage rating of film capacitors is a benefit. Using Aluminum electrolytics here would require complex (and expensive) series stacking with voltage balancing resistors to overcome their voltage rating of 550V or so.

Which is better?

Under ideal conditions, the reliability difference is not huge, although while film capacitors can withstand a 100% overvoltage, electrolytics are often damaged with more than 20% overvoltage. In normal operation, occasional stress can be expected and the self-healing film types cope with this better, not least because electrolytics can short and fail spectacularly, taking down whole banks of components.

Practically, film is easier to use – it is not polarized so cannot be mis-fitted – and the form factor and range of connections allows for easier installation.

Film capacitor types

Within the generic heading of ‘film’ there are many different dielectrics with varying performance [1].

Figure 1

There are multiple film dielectrics available

If losses and reliability under stress are the primary considerations, then Polypropylene (PP) is the best choice due to low dissipation factor (DF) and high dielectric strength. A low DF indicates lower losses and lower heating. However, other types store more charge in a given size or are better at elevated temperatures and Polyester (PET) remains the best choice for low voltage applications.

The structure of PP capacitors

There are two main types of film capacitor construction using metal foil and deposited metallization. [2].

Figure 2

Structure of PP capacitors

For high peak currents, a 5-micron metal foil is inserted between dielectric layers, but this does not have self-healing properties.

Alternatively, a vacuum-formed metallized film can be used. The Aluminum metallization is deposited at high temperature, although Zinc or Aluminum-Zinc alloys are alternatives. In the event of a breakdown, localized heating (up to 6000oC) forms plasma that halts the discharge and repairs the defect. Over time, capacitance will reduce due to this process, allowing the aging of the capacitor to be determined – and replaced if required.

Gross overloads can be addressed by segmenting the film into millions of areas with the current feed to each area acting as a fuse. The additional safety margin allows for a higher voltage rating but does reduce peak current handling.

Paschen curve

PP capacitors have breakdown voltages of several kilovolts due to the 650V/μm dielectric strength of the material. However, with these high voltages, partial discharge (PD) or Corona is a possibility. Micro-voids in the material or air-gaps break down leaving a minute amount of Carbon. Initially this has little impact but can build over time leading to sudden and complete breakdown.

The Paschen curve details this effect and shows the ‘inception’ and ‘extinction’ voltage, with points above the blue Paschen curve being likely to cause PD breakdown.

Figure 3

The Paschen curve details PD breakdowns

There are a few ways to address PD. One is to fill high-voltage capacitors with oil, thereby removing air voids. Alternatively, capacitors can be segmented within the housing to reduce the voltage stress below the inception voltage. Decreasing dielectric thickness is another method that works by decreasing the voltage gradient.

A ‘hybrid’ approach that combines foils and metallization can give a balance between improved peak current capability and the ability to self-heal.

Film capacitor applications

One application example would be a 1kW off-line power supply with 90% efficiency and a power factor corrected (PFC) front end. To account for mains fluctuations, a 20ms ride-through (hold-up) would be required to stop the bus voltage from dropping below the drop-out voltage of 300V, after which the output is compromised.

Figure 4

Application example: 1kW power supply

The energy stored in C1 maintains the output during the hold-up time and the necessary capacitor size is calculated by:

In a practical example, using an Aluminum electrolytic, this would be about 52cm3 (3 cubic inches), such as the TDK-EPCOS B43508 series. Film capacitors would require around 15 TDK-EPCOS EPCOS B32678 in parallel. With a size of 1500 cm3 (91 cubic inches), this is not a viable solution.

If this were an EV, then the bus voltage would be from a battery and the capacitor would only manage the ripple without a hold-up requirement. Typically, this could be 4 V rms from an 80 A rms converter running at 20 kHz. The capacitance becomes:

However, an EPCOS TDK-EPCOS B43508 series 180 μF 450 V electrolytic is only rated for 3.5 A rms ripple current at 60oC. So, 23 would be required in parallel leading to 4140 μF of capacitance with an impractical volume of 1200cm3 (73 cubic inches). Interestingly, this validates the 20 mA/μF ‘rule of thumb’ ripple current rating for electrolytics.

A film option would be the EPCOS B32678 where four in parallel would give a rating of 132 A rms within 402cm3 (24.5 cubic inches) – and this could be reduced further for lower operating temperatures. Film also has the advantage of coping with transient overvoltages better.

If the designer insisted on using electrolytics the inrush current associated with the huge capacitance would have to be managed.

This example is often found in UPS systems, wind and solar power, welding and grid-tied inverters.

Cost is always a factor in any design. For a DC bus derived from 440 V AC, typical costs published in 2013 [3] are:

Other applications: Decoupling and Snubbing

Many power applications, including inverters, require snubbing or decoupling. Generally, film/foil is preferred if there is enough space as metallized devices require special design and manufacturing.

In decoupling applications, the capacitor is placed across the DC rails allowing high-frequency currents to pass through. The capacitor is typically rated at 1μF per 100A switched.

If no decoupling capacitor is fitted, the current circulates through higher-inductance loops, causing transient voltages that can be calculated by:

Even a few nH of inductance can cause significant voltages with di/dt values of 1000 A/μs often happening. PCB traces often introduce parasitic inductances of 1nH per mm, meaning that short traces are essential.

To snub dV/dt across IGBTs or MOSFETs, a resistor/diode network is added to the capacitor.

Figure 5

Snubbing across an IGBT or MOSFET

Snubbing controls EMI by slowing the ringing. It also stops spurious switching, especially with IGBTs, due to high levels of dV/dt. The value of the capacitor can be determined by doubling the sum of the switch output capacitance and mounting capacitance. The resistor value is selected to achieve critical damping of any ringing. Other, optimum, approaches are available – such as those from McMurray [4].

Filtering EMI in mains applications

Due to their ability to self-heal and withstand transients, safety-rated PP capacitors are often used in line-to-neutral mains applications to attenuate differential-mode EMI. These capacitors are required to withstand 4 kV or 2.5 kV transients and are rated as X1 or X2. The various EMC standards would require values of microfarads to achieve standards compliance.

In line-to-earth applications common mode noise can be reduced with Y-type capacitors with 8 kV and 5 kV (Y1 and Y2) transient ratings. Self-resonances remain high due to the low connection inductances of film capacitors, provided connections to the ground/earth are kept short.

Figure 6

Film capacitors can be used to suppress EMI in mains applications

Filtering of motor drives and inverters

As motors are often far from the driver, filtering is required to reduce system EMI to meet overall EMC requirements and reduce voltage stress on cables and motors. PP film capacitors are ideal due to high ripple current ratings, volumetric efficiency and reliability. The inductors and capacitors form a low pass filter and are available packaged together in a single filtering module.

DC-DC converters operate at all power levels and complexities. At some low levels it can make sense to design and ‘make it yourself,’ but at higher powers, a pre-assembled module becomes more cost-effective. New ‘micromodules’ shift that point to lower levels.